1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Wireless communication systems typically include a plurality of base stations, or node-Bs, that provide wireless connectivity to one or more mobile units in associated geographical areas that are usually referred to as cells. Exemplary mobile units may include cellular telephones, personal data assistants, smart phones, text messaging devices, laptop computers, and the like. In 1X Evolution, Data Optimized (1XEVDO) wireless communication system architectures, each mobile unit may transmit messages or signals to one or more active base stations using one or more associated reverse links. Pseudo noise offsets (PN offsets) associated with each of the active base stations are included in an active set list, which is typically stored by a radio network controller coupled to the base stations. Mobile units in a 1XEVDO wireless communication system receive messages and/or signals over a single forward link between the mobile unit and one of the active base stations, which is generally referred to as the serving base station or the serving sector.
Base stations may be added to, or removed from, the active set, a process that is generally referred to as a handoff. For example, a mobile unit may determine that the quality of service of a first reverse link to a first base station has deteriorated and may therefore transmit a request, such as a Route Update message, over the available reverse links to remove the first base station from the active set. The request is forwarded by one or more of the base stations to the radio network controller, which may process the request to remove the first base station from the active set and then forward a Traffic Channel Assignment message to the serving base station. The serving base station may then forward the Traffic Channel Assignment message to the mobile unit over the forward link. The Traffic Channel Assignment message is transmitted over the forward link at a rate determined using a signaling message from the mobile unit that is received on a data request channel of the reverse link. Similarly, the mobile unit may request that a second base station be added to the active set.
The mobile unit will typically select the base station having the strongest forward link as the serving base station. The mobile unit may request a switch of the forward link to other base stations in the active set if, e.g., the quality of the forward link with the current serving base station deteriorates. However, a finite amount of time elapses between transmission of the mobile units request for a switch and the establishment of the forward link between the mobile unit and the new serving base station. Signaling messages may be lost and/or delayed if the signaling message happens to be generated while the switch is underway. For example, if the active set includes first and second base stations, a mobile unit may transmit a Route Update message to add a third base station to the active set. The reverse link to the first base station, which is also the serving base station, has deteriorated and the first base station is not able to receive and/or decode the Route Update message. The second base station receives the Route Update message over the associated reverse link and forwards the Route Update message to a radio network controller. The radio network controller processes the handoff request and sends a Traffic Channel Assignment message to the first (serving) base station. However, the first base station is unable to decode the data request channel of the deteriorated reverse link and so cannot deliver the signaling message to the mobile unit over the forward link.
If the signaling messages are delayed and/or lost during a handoff, the handoff may fail and potentially result in a call being dropped. For example, after sending the Route Update message requesting addition of the third base station to the active set, the mobile unit may recognize that the reverse link to the first base station has deteriorated and may therefore switch to the second base station as the serving base station. The second base station detects the switch and transmits a notification to the radio network controller. However, in order to preserve packet sequencing for transmission of data packets over the forward link, the radio network controller should wait to receive a notification from the original serving sector, i.e. the first base station, before transmitting any traffic or signaling messages to the new serving sector. Since the first base station may not have detected the switch due to the deteriorated reverse link, the first base station may not provided the necessary notification and the attempted switch may time out. Furthermore, if interference causes both the reverse links to the first and second base stations to deteriorate before the third base station can be added to the active set, the call may be dropped.